1. Field of the Invention
This invention relates to a lubricating oil composition having a low coefficient of friction and a reduced copper corrosiveness.
2. Description of the Related Art
In recent years, an increase in the output of internal combustion engines such as automobile engines has caused individual parts of the engine, for example valve operating systems and cylinders, to be exposed to high temperatures. Moreover, the number of contacts per unit time of metals with each other has been increased, thus placing the internal combustion engine under severe operating conditions. Lubricating oils for internal combustion engines must function under severe operating conditions. With a reduction in engine size and an increase in the performance which results in an increase in the number of revolutions and an increase in the output, engine oils are required to be versatile and possess high levels of performance.
Examples of fundamental performances required of the engine oils include detergency, dispersancy, a reduction in the friction, prevention of abrasion and seizing, reduction of thermal and oxidative deterioration, reduction of corrosion, and cooling and sealing functions. For example Japanese Patent Publication No. 23595/1991 proposes a lubricating oil composition particularly useful in the reduction in the mechanical friction loss of four-cycle engines, said lubricating oil composition comprising a mineral oil and/or a synthetic oil having a kinematic viscosity at 100 degrees C. of 3 to 20 cSt and, incorporated therein, (a) 0.2 to 5% by weight of sulfurized oxymolybdenum organophosphorodithioate (hereinafter abbreviated to "MoDTP" and/or molybdenum dithiocarbamate (hereinafter abbreviated to as "MoDTC"), (b) 0.1 to 7% by weight of zinc dithiophosphate, (c) 0.1 to 20% by weight of calcium alkylbenzenesulfonate and (d) 1 to 15% by weight of an alkenylsuccinimide and/or a boron compound derivative of an alkenylsuccinimide. According to this lubricating oil composition, the coefficient of friction in the mixed/boundary region can be reduced to about 1/3 of that for the conventional engine oil.
One of the important features of the lubricating oil is that the lubricating oil does not attack a metal present within the engine during use. Free sulfur, sulfur compounds, or acidic substances are considered to cause corrosion. Since a copper plate is most sensitive to these substances, the corrosion of a copper plate when exposed to lubricating oil is evaluated as a measure of the corrosiveness of the lubricating oil. It is a common practice to add nitrogenous metal deactivators, such as benzotriazole, to the lubricating oil for the purpose of reducing the copper corrosiveness. However, the addition of these metal deactivators in a large amount results in hardening of sealing rubbers.
On the other hand, when MoDTP or MoDTC is added in a relatively large amounts of 0.2 to 5% by weight as disclosed in the above-cited Japanese Patent publication for the purpose of reducing the friction, the copper corroding activity is unacceptably increased.
Further, lubricating oils containing organomolybdenum compounds, such as MoDTP or MoDTC, have a problem of a high coefficient of friction at an early stage, i.e., at the stage of running-in. The additive to the lubricating oil is adsorbed on the surface of the metal to form a boundary lubrication film which serves to reduce boundary friction. However, a relatively substantial amount of time is taken for the organomolybdenum compounds to be adsorbed on the surface of the metal to develop the effect of reducing the friction.
When lubricating oil compositions containing the organomolybdenum compounds are used as an engine oil, the effect of reducing the friction develops after running a distance of 2000 to 3000 km, although this depends upon the running conditions of automobiles. However, after the above-described running, the time for the development of the effect of reducing the friction often overlaps with the time for the replacement of the engine oil. In such a case, an increase in the amount of addition of MoDTP or MoDTC does not lead to the development of the effect of reducing the friction at an earlier stage and rather increases the copper corrosiveness.
It would be desirable to provide a lubricating oil composition having a low coefficient of friction and a reduced copper corrosiveness. It would also be desirable to provide a lubricating oil composition which exhibits a low coefficient of friction from an early stage and has reduced copper corrosiveness.